Motion sensors find widespread use in many industrial, commercial, and consumer markets. For industry and commerce, a primary use is for security by detecting human motion in a factory, office, or home. For commercial establishments, such as department or grocery stores, motion sensors are used to open doors automatically upon sensing the approach of a person or other moving object. Other uses are varied, including the counting of objects on a conveyor belt.
Motion sensors presently available include mechanical switches, magnetic sensors, photoelectric sensors, acoustic sensors, microwave sensors, and active and passive infrared sensors. Each of these technologies has strength and weaknesses. Mechanical switches that are commonly found in doormats of grocery store doors are subject to wear from heavy and continual traffic thereon. Moreover, the doormats are often in an entry location and exposed to extreme weather such as heat, rain, and snow which affects their reliability and lifetime. A further drawback of mechanical switches is that they require physical contact with the object to be sensed.
The other types of sensors mentioned above do not require physical contact with the moving object but have their own drawbacks. Magnetic sensors, for example, have a very short range and can detect only ferrous or magnetic materials. Acoustic sensors cannot be narrowly focused on a selected region of space and furthermore are relatively expensive, quite large, and unable to adjust adequately to changes in background noise. Microwave sensors suffer from cross interference with adjacent microwave sensors and from wide beam dispersion. Photoelectric sensors, on the other hand, have too narrow a beam dispersion and require that the moving object break the beam. Although this may be an adequate technique for opening doors, it can be easily circumvented by one engaged in unauthorized entry.
Recently much interest has been shown in infrared motion sensors. Such sensors take two forms: active sensors that include one or more emitting elements and one or more receiving elements and passive sensors that comprise only receiving elements that sense a change in ambient infrared energy. Passive infrared sensors are expensive and are easily fooled by changes in ambient conditions. Furthermore, since they detect such energy from the body heat of a person, they may not detect persons wearing heavy clothing that trap body heat.
A drawback of active sensors is their reliance on a plurality of spaced apart detectors and emitters. The additional emitters are necessary to provide sufficient intensity for the detector to discriminate against ambient light. The higher intensity, however, limits the sensitivity of the detector and thus the dynamic range of the sensor. A more serious drawback is the lack of automatic adjustment in response to changes in ambient conditions. For example, if a surface within the sensing region, such as a sidewalk, changes its reflective characteristics because of snowfall or rain, the amount of reflected infrared energy will change, causing the detector to falsely indicate the presence of an object. The same problem occurs if an object such as a product display in a store is moved into the sensing region. The active sensor must be manually adjusted to compensate for the presence of the new object. Otherwise, the sensors are overly sensitive to changes in energy and can easily saturate unless the energy received is decreased. This adjustment of sensitivity, however, limits the range in which the sensor is effective.